The compute(Key, BiFunction) method of ConcurrentHashMap class is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping is found).
- This method is used to atomically update a value for given key in ConcurrentHashMap.
- If the remapping function throws an exception, the exception is re-thrown, and the current mapping is left unchanged.
- During computation, update process on this map from other threads has been blocked so the computation must not attempt to update any other mappings of this Map
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For example, This mapping append string value of mapping:
ConcurrentHashMap.compute(key, (key, value) -> (value == null) ? msg : value.concat(msg))
Syntax:
public V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
Parameters: This method accepts two parameters:
- key: key with which the value is to be associated.
- remappingFunction: function to do the operation on value.
Returns: This method returns new value associated with the specified key, or null if none.
Exception: This method throws folloeiong exceptions:
- NullPointerException: if the specified key or remappingFunction is null.
- llegalStateException: if the computation try to do a recursive update to this map that would otherwise never complete.
- RuntimeException: if the remappingFunction does so, in which case the mapping is unchanged.
Below programs illustrate the compute(Key, BiFunction) method:
Program 1:
// Java program to demonstrate // compute(Key, BiFunction) method. import java.util.concurrent.*; public class GFG { // Main method public static void main(String[] args) { // create a ConcurrentHashMap and add some values ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>(); map.put( "Book1" , 10 ); map.put( "Book2" , 500 ); map.put( "Book3" , 400 ); // print map details System.out.println( "ConcurrentHashMap: " + map.toString()); // remap the values of ConcurrentHashMap // using compute method map.compute( "Book2" , (key, val) -> val + 100 ); map.compute( "Book1" , (key, val) -> val + 512 ); // print new mapping System.out.println( "New ConcurrentHashMap: " + map); } } |
ConcurrentHashMap: {Book3=400, Book1=10, Book2=500} New ConcurrentHashMap: {Book3=400, Book1=522, Book2=600}
Program 2:
// Java program to demonstrate // compute(Key, BiFunction) method. import java.util.concurrent.*; public class GFG { // Main method public static void main(String[] args) { // create a ConcurrentHashMap and add some values ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(); map.put( 1 , "Kolkata" ); map.put( 2 , "Nadia" ); map.put( 3 , "Howrah" ); // print map details System.out.println( "ConcurrentHashMap: " + map.toString()); // remap the values of ConcurrentHashMap // using compute method map.compute( 2 , (key, val) -> val.concat( " (West-Bengal)" )); map.compute( 3 , (key, val) -> val.concat( " (West-Bengal)" )); // print new mapping System.out.println( "New ConcurrentHashMap: " + map); } } |
ConcurrentHashMap: {1=Kolkata, 2=Nadia, 3=Howrah} New ConcurrentHashMap: {1=Kolkata, 2=Nadia (West-Bengal), 3=Howrah (West-Bengal)}
Program 3:To show NullPointerException
// Java program to demonstrate NullPointerException // for compute(Key, BiFunction) method. import java.util.concurrent.*; public class GFG { // Main method public static void main(String[] args) { // create a ConcurrentHashMap and add some values ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(); map.put( 1 , "Kolkata" ); map.put( 2 , "Nadia" ); map.put( 3 , "Howrah" ); try { // remap the values of ConcurrentHashMap // using compute method map.compute( null , (key, val) -> val.concat( " (West-Bengal)" )); } catch (NullPointerException e) { // print Exception System.out.println( "Exception: " + e); } } } |
Exception: java.lang.NullPointerException
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